Induced polarization signal manifestation in multi-spacing installations in offshore areas up to 100 m deep

Abstract

. The purpose of the work is to show the manifestation of an induced polarization signal in the transient electromagnetic signal for multi-spacing axial electrical installations depending on the spacing and sizes of the source at different depths of installation for the offshore conditions of sea depth of up to 100 m. The study uses the solution of the direct problem of a transient electromagnetic field for conducting polarizable media with a description of electrical resistivity dispersion by the Cole – Cole formula. Analysis is given to the change in the transient signal Δ U ( t ), final difference of the transient signal Δ 2 U ( t ) and transform P 1( t ) (ratio of Δ 2 U ( t ) to Δ U ( t )) depending on multi-spacing installation size. The study involves installations with a source length (a source is a horizontal grounded electrical line AB) from 50 to 500 m, receiver length (receiver is represented by three-electrode electrical lines) from 50 to 500 m, and distance between the centers of the source and receiver (spacing) multiple of the source length: (3/2)·AB, 2·AB, (5/2)·AB, 3·AB, (7/2)·AB, 4·AB, (9/2)·AB, 5·AB. Comparison is given to the signals from conductive model and conductive polarizing model. A multi-spacing installation was placed inside a conductive medium with a conductive polarizing base. The conductive medium was associated with the layer of sea water in offshore areas with sea depths of up to 100 m. The conductive polarizing base was represented by a geological formation (ground) covered by a layer of water. Calculations performed as a result of conducted research works show the manifestation of various components of the transient process associated with electromagnetic field formation and manifestation of low-frequency dispersion of the electromagnetic properties of the earth caused by both galvanic and eddy currents. These components manifest themselves in different ways on multi-spacing installations at different depths. Therefore, it could be argued that the components of the transient process associated with the transient electromagnetic field, galvanically induced polarization and inductive induced polarization manifest themselves in different ways in multi-spaced installations of different sizes immersed at different depths. Induced polarization manifests itself in two ways for water area conditions as it is associated with both galvanic and eddy currents. Previously, when performing practical measurements, the manifestation of inductive induced polarization was considered as interference manifestation. But being simulated this signal can be considered as information about induced polarization. The factor influencing the manifestation character of induced polarization signal in the transient signal is the installation height above the bottom Δ h and the spacing r. Δ h is the distance between the installation and the seafloor, which is a polarizing base of the model. r is the distance between the centers of the source and the meter represented by a three-electrode measuring line. Depending on the installation height and spacing the induced polarization signal in the transform P 1( t ) can appear as an ascending branch at later times, as well as in the form of a descending branch that turns into negative values of P 1.